During operation of an internal combustion engine, hydrocarbon fuel and oxygen burn in the presence of nitrogen. The fuel is converted principally into carbon dioxide and water, creating extremely high gas pressures that displace pistons and produce engine power. This combustion also results in the formation of contaminants that include organic, sulfur and nitrogen-based acids as well as soot formed from incomplete combustion. These contaminants cause undesirable engine wear, corrosion, increased oil viscosity and unwanted deposits when introduced into the lubricating oil through contact in the cylinder bore or through blow-by gases. Increases in corrosion, wear and viscosity degrade engine performance. Deposits on or near the pistons allow lubricant to pass the piston rings where it burns in the combustion chamber, generating a commensurate economic loss. Piston deposits also allow combustion gas to blow by piston rings, bringing additional acid and soot into the lubricant.
Lubricant additives, particularly detergents and dispersants, are used to combat these problems. Detergents are effective for controlling piston deposits; dispersants are effective for controlling viscosity increase due to soot and sludge formation; and both detergents and dispersants are effective for neutralizing combustion acid. However, these additives do have limitations. First, as detergents and dispersants neutralize combustion acids, they are stored in the engine lubricant as acid-base complexes or salts in the form of soluble or dispersible species. Solubility of these species limits the capacity of the lubricant to store such relatively polar products. If the upper solubility limits are surpassed, some of these polar by-products may precipitate, adhere to pistons, and form deposits. For example, Alan Schetelich and Pat Fetterman have reported in SAE Paper #861517 (October 6-9 International Fuel & Lubricants Meeting) that at a high detergent level in a diesel engine, up to 35% of the piston deposits were derived from the detergent. Clearly, increasing detergent concentration has diminishing returns. Second, high dispersant concentrations increase the viscosity of the lubricant, especially at low temperature, and high viscosities decrease lubricant and engine efficiency. While dispersants typically have higher solubility limits than detergents, they are more expensive. Thus, viscosity and economics limit how much dispersant can be added to the lubricant. Third, both detergents and dispersants are stoichiometric additives. Unlike a catalytically active material, each molecule performs its function one time and has a defined, limited capability.
As engine technology progresses toward greater cleanliness and efficiency, lubricants and additives face additional limitations. One such engine improvement, Exhaust Gas Recirculation (EGR), burdens the lubricant and additives with added levels of soot and acid, especially in diesel engines. While EGR decreases emission of undesirable species to the environment, it also operates at higher temperatures and, as a result, degrades the lubricant and additives more quickly. In a gasoline engine improvement, additional acid forms as combustion temperatures are increased in a quest for better fuel economy.
Further, certain components within the lubricant additives foul exhaust after-treatment systems and limit their effectiveness. These components—sulfated ash, phosphorus and sulfur (SAPS)—are introduced into these systems through the combustion of the lubricant. One such after-treatment system, a Diesel Particulate Filter (DPF), removes solids from diesel engine exhaust gas. These particulate filters capture fines and are regenerated by burning off trapped materials. However, non-combustibles (detergent and metallic anti-wear additives) from the lubricant accumulate over multiple cycles and foul the filter. Analytical procedures performed on the lubricant for SAPS accurately predict its potential to contribute to this fouling problem. Another exhaust gas after-treatment system removes nitrogen acids (NOx) from diesel engines. Lubricant-derived SAPS partially poison this system and reduces its effectiveness.
Such after-treatment mechanisms are required to meet national emission limits and have specific performance requirements. For example, the United States Environmental Protection Agency mandates that all heavy-duty DPFs must operate for 150,000 miles before cleaning or replacement. As a result, limits on SAPS in commercial lubricants have been set by organizations that establish lubricant standards.
To avoid the problems outlined above, several additives must be reduced or replaced in a careful balance to maintain performance. For example, zinc dialkyldithio phosphate (ZnDDP) functions in two ways when used as a lubricant additive—as an anti-wear agent and as an antioxidant—and its concentration is determined by both roles. ZnDDP, however, also poisons emission catalysts through its phosphorus content. Therefore, any reduction in its concentration to avoid impacting exhaust after-treatment systems may require augmentation of either non-SAPS containing antioxidants or anti-wear agents. Other additives also serve as the source for lubricant-derived SAPS and may have to be reduced or eliminated to prevent after-system treatment fouling. For example, detergents contain sulfur and metals that give rise to sulfated ash.
Thus, while soot and acid derived from EGR and higher temperatures further contaminate the lubricant, other emission reduction technologies require a reduction in concentration of some additives intended to mitigate these by-products. Within the current paradigm of lubricant formulation, the only way to both reduce detergent level in the lubricant and adequately neutralize the increased amount of acid entering the lubricant is to decrease the oil drain interval. However, this approach has a severe economic penalty. Frequent oil drains are undesirable and have both direct and indirect consumer costs, as well as environmental impact. For each oil drain, consumers bear the direct costs of a new filter and lubricant, mechanic labor, and in the case of commercial trucks, lost delivery time. Consumers bear the indirect costs of filter and lubricant recycle or disposal. They also endure the negative environmental impact associated with the inappropriate disposal of used engine oil. Extended oil drain intervals instead conserve valuable resources. Since lubricant additive levels, in general, determine the oil drain interval, performance specifications pressure the lubricant industry to maintain upper limit concentrations of additive. In addition, they must also maintain backward compatibility to ensure that new formulations perform adequately in older engines.
Prior art patents to Brownawell et al. (U.S. Pat. Nos. 4,906,389, 5,068,044, 5,164,101 and 5,478,463) teach that immobilizing a strong base in an oil filter will reduce piston deposits, and pending U.S. patent application Ser. No. 11/133,530 teaches how to optimize the strong base for maximum acid retention capability. These disclosures represent one potential approach to deal with deposits, but if used with conventional lubricants, do not solve the broader issues outlined above. There is clearly a need for improved approaches to engine lubrication.
In light of the foregoing, there still remains a need for a lubrication system that significantly reduces the SAPS levels in a lubricant without negatively affecting engine performance. In particular, a lubrication system is desired that minimizes the use of SAPS-containing additives that combust to form contaminants which foul emissions after-treatment systems. The present invention addresses these needs in the art.